The present invention relates to apparatus and process for separating a mixture of solid particles into separate light and heavy fractions. The separation system of the invention, while not limited thereto, finds particular use in the scrap metal industry where it can be beneficially applied for the separate recovery of light and heavy metal fractions from crushed scrap, substantially freed of non-metallic components.
It has been known since early times that dry granular materials could be separated by dropping a mixture of such materials through a transverse air blast whereby the trajectory of the falling particles was differentially modified and the lighter particles blown by the air stream through a greater horizontal distance were thus separately recoverable from the heavier materials. Early examples of such devices are illustrated by U.S. Pat. No. 775,965 (1904) proposed for separation of gold from accompanying gravel; and in later U.S Pat. No. 2,791,331 proposed for cleaning gravel to free the same of dirt and light trash or other undesired contaminants. Means are commonly provided in separators of this type for adjusting the feed rate of the granular materials to the separator and the velocity of the air stream and/or the size of the air inlet opening into the separator.
In certain devices of the type described, instead of using positive blowers, the movement of the traversing air stream is effected by suction applied at the air outlet of the separator, as seen for example in U.S. Pat. Nos. 3,005,547 and 3,441,134. The use of a substantially closed air circulating system wherein the air and lightest materials are withdrawn overhead and the air recycled by admission into the suction inlet of a blower discharging to the separator inlet, is disclosed in U.S. Pat. No. 3,655,043.
It has also been proposed, see for example U.S. Pat. No. 1,877,861, to direct a stream of air carrying granular solids into a closed separating chamber communicating with an overhead suction line, wherein the heavier granules drop out of the ascending air stream under the influence of gravity, while the lighter materials are withdrawn overhead into the suction line passing enroute through a cyclone separator in which further separation of the components in the ascending air stream may take place. Other modifications of systems for density separation of solids by an air stream have been described in the art, as for example in U.S. Pat. No. 3,311,234, which features projecting the granular solids into the air stream at a positive inlet velocity normal to the direction of the higher velocity air stream.